1. Field of the Invention
The present invention relates to a method of detecting the density of toner deposited on an image recording member, and a toner image transferring type image forming method and image forming apparatus for which the toner density detecting method is employed.
2. Description of Related Art
In using a toner image transferring type of image forming apparatus such as an electrophotograchic copying machine, a laser printer, etc., an uniform amount of toner must be always stuck on the photosensitive member in order to keep forming quality images in density and chromaticity. The amount of toner stuck on the photosensitive member varies according to the electro static charge on the toner which also varies mainly according to circumstances such as humidity, temperature, etc. A feedback system, as for 1) forming a test toner image on the photosensitive member, 2) measuring the density of toner deposited on the photosensitive member optically and 3) transmitting the measured density to a controller for determining a toner density in the developer and a controller for determining a developing bias voltage, has been proposed in various forms. In measuring the density of toner deposited on the photosensitive member with a photosensor, however, errors are caused by changes of the photosensor in characteristic due to fatigue and smudges on the photosensor (mainly due to toner smoke).
U.S. Pat. No. 4,313,671 discloses a second photosensor for measuring reflected light from the surface of the photosensitive member as well as a first photosensor for measuring the density of toner deposited on the photosensitive member, and it also teaches that the measurement with the first photosensor is evaluated in accordance with the measurement with the second photosensor and that the evaluated value is used for controlling a toner supply Even with this arrangement, if the Photosensitive member has a stain or a scratch, errors are apt to occur. In detecting the density of color toner deposited on the photosensitive member by measuring the amount of diffused light, the output of a sensor which measures the amount of diffused light from the photosensitive member is too small to detect the density accurately.
In a conventional feedback system, either a value obtained from only one test toner image or a value obtained from a few test toner images which are formed under the same conditions is used for determining a developing bias voltage (refer to Japanese Patent Laid-Open Publication No. 56-33661). When the toner density/densities of the test toner image(s) is/are in either so low a level or so high a level that the toner density/densities can not bring about variations in measurements, (an) error(s) in measuring the toner density/densities is/are caused, and nothing compensates for the error(s). Consequently, it is very difficult to determine an optimum developing bias voltage accurately.
According to a conventional feedback system, the detection of the toner density of a test toner image is performed, every time an image is to be formed, immediately after the machine is supplied with power, or immediately before a multiple copying operation is started. A machine in which a system of detecting the toner density of a test toner image every time an image is to be formed is employed is inferior in copying speed. Especially when such a system is applied to a full-color copying machine wherein each of the developing devices is set to a developing position before each developing operation, the copying speed will be quite slow. From this point of view, this type of machine should be provided with such a feedback system as the toner density of a test toner image is detected immediately after a power supply for the machine and after the completion of a multiple copying operation, and the detected value is applied to the next copying operation. With this arrangement, however, when the next copying operation is started a long time after the former operation was finished, circumstances (humidity, temperature, etc.) may change, and the developing bias voltage determined in accordance with the value measured long ago may be improper in the current circumstances.